Various types of nets are used by commercial fishing vessels to harvest fish from the sea. Nets such as the purse seine are designed to be towed behind a trawler to trap the fish that are in the path of the moving net within a sock formed when the bottom of the net is drawn closed. In contrast, gill nets are typically deployed or laid out behind a fishing boat in a "J" pattern, but are not towed through the water after being deployed. Instead of seining the fish from the water with a moving net, the fish are ensnared when they swim into the gill net as it drifts behind the boat. The mesh of a gill net is sized so that only fish larger than a minimum size are taken. The heads of the fish pass through the mesh, and their gills will become entangled. Fish that are too small to be taken pass through the mesh unharmed. The gill net is subsequently drawn on board the boat using a power wench, and the fish that are entrapped in the mesh are manually extracted for storage on the boat until they are processed.
Gill nets are typically about one-quarter mile in length. The width of a gill net may be limited by the fishing regulations applicable in a particular commercial fishing zone. For example, in Bristol Bay, Canada, the nets are limited to 13 feet in width, while in the U.S., nets up to 60 feet wide are commonly used.
A float line is threaded through the top mesh of a gill net and typically supports floats spaced apart at about two foot intervals. A nylon sheathed lead line is normally threaded through the bottom mesh of the net, so that the net floats as a sheet, extending from the surface downwardly in a vertical orientation, with the lower edge of the net either dragging on the bottom or clear of it.
Fish that see a net will typically attempt to swim around it. Due to the length of a gill net, most fish attempting to avoid the net may try to swim above or below it. The width of the net and its visibility to the fish thus directly affect the number of fish that succeed in avoiding entanglement in the mesh by swimming around the net. A conventional gill net is generally a single color. The manufacturer may select a color for the net so that it is less visible to fish in water. However, algae, plankton, suspended sediment in the water, and depth can change the light absorption characteristics of sea water, and thus its color, so that a gill net that is colored to blend well with the water in one location may be substantially more visible to fish in the water at another location. Ideally, a gill net should be colored to be virtually invisible to the fish in the water so that they swim into the net and become entangled in its mesh. However, changing color absorption characteristics of sea water can make this a difficult goal to achieve under all conditions. Alternatively, it would be desirable to induce fish to swim into a portion of the net that is less visible than other portions of the net that are more visible, which the fish are attempting to avoid.